1. Field of the Invention
The present invention relates to a magnetic recording medium which allows high density magnetic recording and a method for manufacturing the same.
2. Related Art
In recent years, multimedia for data such as image, video, sound progresses, which results in increase in information or data volume of retrieval data per one user. For this reason, mass volume of database and fast processing are required. On the other hand, according to improvement of surface recording density of a magnetic recording medium due to increase in recording volume of HDD (Hard Disk Drive), each recording bit size on the magnetic recording medium becomes extremely fine such as several tens nanometer. In order to obtain a reproduction output from such a fine recording bit, it is required to secure large saturation magnetization and film thickness as much as possible for each bit. However, fineness of the recording bit makes a magnetization amount per bit small, which causes such a problem as disappearing of magnetized information or data due to magnetization reversal due to “thermal fluctuation”.
Generally, the influence of the “thermal fluctuation” becomes larger as the value of Ku·V/(k·T) becomes smaller. Here, Ku represents magnetic anisotropy energy density, V represents magnetizing minimum unit volume, k represents Boatman's constant, and T represents absolute temperature. Experiential, it is said that, when Ku·V/(k·T) becomes less than 100, magnetization reversal due to the “thermal fluctuation” occurs. That is, the magnetic anisotropy energy required for maintaining orientation of magnetization of magnetic particles in one direction is expressed by a product of the magnetic anisotropy energy density Ku and the volume V of the magnetic particles, but when Ku·V/(k·T) becomes less than 100, the magnetic anisotropy energy required for maintaining orientation of magnetization of magnetic particles in one direction becomes about thermal fluctuation energy at the room temperature. Therefore, such a phenomenon occurs that magnetization fluctuates according to time elapsing and recorded information or data disappears.
In a magnetic recording medium of a longitudinal magnetic recording system, since a demagnetizing field in the recording bit in a high recording density region becomes strong, magnetized information or data is apt to be influenced by the “thermal fluctuation” even while a magnetic particle diameter is relatively large. On the other hand, in a magnetic recording medium of a perpendicular magnetic recording system, since the magnetized minimum unit volume V can be made large while particle diameter on a medium surface remains small by causing magnetic particles to grow in a direction of the film thickness, the influence of the “thermal fluctuation” can be suppressed. However, when high density of the magnetic recording medium further advances in the future, a limit will appear in robustness to the “thermal fluctuation” even in the perpendicular magnetic recording system.
A magnetic recording medium called “a patterned media” is getting an attention as a medium for solving the problem of resistance to this thermal fluctuation (refer to Japanese Patent Laid-Open No. 2001-176049 (FIG. 1), for example). The patterned media is, generally, a magnetic recording medium on which a plurality of magnetic regions serving as a recording bit unit are respectively formed independently in a non-magnetic layer, but it can be said to be a medium where a magnetically continuous magnetic thin film is partitioned to each recording magnetic domain size. In an ordinary patterned media, for example, such an oxide as SiO2, Al2O3, TiO2, or such a nitride as Si3N4, AlN, TiN, such a carbide as TiC, or such a boride as BN is used as a non-magnetic layer, and a ferromagnetic region is selectively formed in each of these non-magnetic layer.
Since the patterned media is obtained by partitioning a magnetic thin film to respective recording magnetic domain size, the magnetized minimum unit volume V can be made large, so that the problem about the thermal fluctuation can be avoided. In a conventional continuous magnetic thin film, a magnetic domain has about 1000 grains per one bit as the number of magnetic particles. However, according to advance of high recording density, the number of gains corresponding to one bit decreases. As a recording mark edge depends on a grain boundary of a gain, it becomes necessary to make the grain small as much as possible in order to secure S/N ratio. Accordingly, V must be made small in the conventional continuous film. However, in the patterned media, since an edge of the recording magnetic domain can be defined by a structure thereof, improvement in S/N ratio can be expected without making V small.
In the pattern media, ferromagnetic regions, each being a recording bit unit, are independent from one another, interference between respective recording bits can be prevented, which is effective in prevention of disappearance of record due to an adjacent bit or reduction of noises. Further, a domain wall movement resistance increases due to patterning, so that improvement of magnetic characteristics can be enhanced.
On the other hand, when a track density is improved, a problem about interference with an adjacent track is developed. In particular, reduction of writing blurring due to a recording head magnetic fringe effect is a important technical problem. Now, a discrete track media which separates recording tracks from each other physically has been proposed (refer to Japanese Patent Laid-Open No. 07-85406 (FIG. 1), for example). When the discrete track media is used, a side erase phenomenon at a recording time, a side read phenomenon where information or data pieces of adjacent tracks are mixed at a reproduction time or the like can be reduced. Since it is possible to increase a density in a track direction largely, a magnetic recording medium with a high density can be provided.
Since the patterned media must be separated in both a recording track direction and a recording line direction physically, a high level nanometer working technique is required. Since the discrete track media performs separation only in the recording track direction, patterning on the discrete track media is easier than that on the patterned media.
As described above, since the patterned media can suppress magnetization reversal due to the “thermal fluctuation”, it is effective as the high density magnetic recording medium. Further, since the discrete track media can increase a density in the track direction, it is effective as the high density magnetic recording medium. However, when record is written in each of the patterned media and the discrete track media, magnetic field is concentrated on a corner of a recording track (or a recording bit) so that electrostatic discharge occurs due to sparking. Such a problem arises that a magnetic material deteriorates due to such a local electrostatic discharge, life of each of the patterned media and the discrete track media (hereinafter, called “patterned media, too) is shortened.
There is also a problem that the patterned media is inferior in lubricant application to the conventional continuous film. The lubricant (usually, perfluoropolyether) includes a portion (absorbing layer) which chemically bonds carbons formed on a magnetic recording medium surface and a portion (a free layer) which does not cause chemical bonding and is movable freely. The bonding layer becomes important for suppressing decrease/loss of lubricant due to contact or the like of the recording/reproducing head and preventing spinning-off due to centrifugal force at a disc rotation time. The free layer becomes important for flowing to a portion where the lubricant has decreased to cover the same to self-repair. In the patterned media, since the surface area of carbon formed as a protective film becomes large, the absorbing layer of the lubricant increases and the free layer decreases. Therefore, when the patterned media is used for a long period, the self-repair owing to the free layer does not overtake decrease of lubricant, so that the life of the patterned media is reduced.
Furthermore, as the cause of life reduction of the patterned media, there is an abnormal protrusion on the medium surface. In addition to such a risk that the abnormal protrusion and the recording/reproducing head collides with each other to cause failure, the recording/reproducing head and the magnetic recording medium degrades due to influence of heat generated by the collision. In the patterned media, a hard mask (usually, metal such as Ti or Ta) is used for working a magnetic material, but it is difficult to remove the hard mask and a portion of the hard mask which can not be removed causes the abnormal protrusion.